This invention is concerned with improvements in or relating to blind riveting. The expression "blind riveting" is used herein to denote a procedure in which a tubular rivet inserted into a hole in a workpiece from one side is set by radial expansion of a tubular portion of the rivet at the other side of the workpiece by pulling through the rivet a head of a mandrel which is too large to pass through the tubular portion of the rivet without deforming it. Setting of a rivet in blind riveting by customary procedures sometimes results in the mandrel breaking, the mandrel head being either retained in the set rivet or falling away at the blind side of the workpiece, or by the mandrel being pulled right through and clear of the rivet so that the mandrel can be used again and again. In the latter case, commonly referred to as "pull-through blind riveting", it is customary to use a long mandrel with a number of, for example 25, rivets on it, the mandrel being mounted in a suitable tool and the rivet set upon reciprocation of the mandrel and forward feeding of the rivets.
Blind pull-through mandrel rivets are disclosed, for example, in British patent specification No. 1,323,873 based on an application filed July 28, 1969. The use of expendable mandrels in blind riveting, that is to say where the mandrel breaks and can therefore by used for only one setting operation, is relatively expensive, but the performance of the set rivet, including the ability of the rivet to pull together the parts of the workpiece and clench them tightly, is usually considered superior to that which has yet been achieved with pull-through blind riveting. There are, however, many occasions where the performance of a rivet set by pull-through blind riveting is adequate for the particular purpose it is to serve, provided that a high standard of consistency and reliability can be achieved.
In blind riveting, where two or more sheets are to be riveted together, it is necessary to ensure that radial expansion of the tail-end of the rivet takes place to engage the blind side of the workpiece before sufficient expansion of an intermediate portion of the rivet shank occurs to hold the sheets apart, and that radial expansion and longitudinal contraction of the intermediate portion of the shank takes place subsequently. With some shapes of rivet, the size of the mandrel head necessary to achieve this effect is such that when it is pulled right through the rivet it is likely that a small amount of material of the wall of the rivet will either be completely removed from the rivet or will appear as a burr around the rivet wall projecting in an unseemly manner from the set rivet after completion of the operation. If such material remains attached to the rivet its appearance is unsightly; if it is removed by the mandrel, it will fall off at some unpredictable time and possibly become trapped inside the rivet-setting tool, which can become especially troublesome if the tool being used is one with hydraulic seals.